The metabolism of the corticosteroid side-chain is being studied in man and animals. This is accomplished by utilizing corticosteroids labelled with tritium at position 21 and C14 at position 4 ((4-C14-21-3H)- corticosteroids). The objectives are to determine the significance of a pathway of degradation which leads to the formation of steroidal carboxylic acids and loss of tritium of the methylol group. The steroidal 20-hydroxy-21-oic-acids derived from cortisol, named the cortoic acids are excreted into the urine. The enzyme, 20-hydroxy acid synthetase, catalyzes the formation of the cortoic acids. Other enzymes, the ketoaldehyde dehydrogenases, catalyze synthesis of steroidal 20-keto acids. BIBLIOGRAPHIC REFERENCES: Monder, C., B. Zumoff, H.L. Bradlow, and L. Hellman: Studies on the biotransformation of cortisol to the cortoic acids in man: Metabolism of 21-dehydrocortisol. J. Clin. Endocrin. & Metab. 40: 86 (1975). Monder, C. and A. Coufalik: Activities of tyrosine, alanine and aspartate aminotransferases of fetal rat liver in organ culture. Enzyme 20: 111 (1975).